Execute these Queries in you database
Alter table oc_currency change column symbol_left symbol_left varchar(50);
Update oc_currency set symbol_left='<i class="fa fa-inr"> </i>' where currency_id=4;
Change the stylesheet.css in "catalog/view/theme/default/stylesheet/stylesheet.css" To remove the font size to make it even across the application
Note by default Font Awesome is included in 2.0 Opencart if your theme doesn't have added it to make this solution work
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